1. Field of the Invention
The present invention relates to a method for producing an asymmetric support membrane for forming a composite membrane, and the asymmetric support membrane having a narrow pore size distribution and an improved permeance and mechanical strength. More particularly, the present invention relates to a method for producing an asymmetric support membrane using a phase separation method, which comprises fixing a structure of a polymer solution using a physical gelation property to easily control pore size and by which producing an asymmetric support membrane having a narrow pore size distribution and also a high permeance and mechanical strength; and the asymmetric support membranes obtained thereby.
2. Description of the Prior Art
The various methods for producing an asymmetric support membrane using a phase separation have been known in the art for some time.
A method for producing an asymmetric support membrane using a phase separation was carried out by Loeb and Sourirajan in U.S. Pat. No. 3,133,132. This method comprises dissolving a polymer as a membrane material in a suitable solvent to form a polymer solution and then immersing it into a non-solvent to bring about a phase separation to form an asymmetric support membrane in a flat plate or hollow fiber form.
Generally, the polymer solution exists as a network form in which the polymer chains are entangled with one another as the concentration increases. In order to produce the desired form of membrane, the polymer solution must have a sufficient viscosity and flowability so as to form the membrane.
However, if the flowability is too high, the entanglement of the physically formed network structure of the polymer chains becomes weak. Thus, when the polymer solution is met with a non-solvent to bring about a phase separation, i.e. when the non-solvent enters into the polymer solution and the solvent therein is flowed off onto the non-solvent, macrovoids are formed and a membrane having a huge pore size is produced by the thermodynamic or kinetic property. Thus, the mechanical strength of the membrane is weakened, or the membrane permeance is decreased due to the rapid phase separation and the shrinkage during the dryness, or defects are produced.
As described above, the polymer solution generally exists as a network structure having uniform distance that the polymer chains are entangled with one another. If the network structure of the polymer solution is physically fixed and maintained when producing an asymmetric membrane using a phase separation, a membrane having the fixed form of the network structure is produced. The membrane can be converted into a membrane of sponge structure having a homogeneous pore size and a high permeance by means of a phase conversion procedure.
Accordingly, it is an object of the invention to provide a method for producing an asymmetric support membrane for a composite membrane by a phase separation, which comprises fixing a structure of a polymer solution using a physical gelation property to easily control a pore size. The membrane as obtained has a narrow pore size distribution, and an improved permeance and mechanical strength.